1. Field of the Invention
The invention relates generally to a method and apparatus for providing a composite video image for display on a monitor, and specifically to a method and apparatus which permit first and second video signals having different line and/or frame frequencies to be selectively displayed on the monitor.
2. Background Information
In previously known systems, in order for a video signal having a first frequency to be delivered to a monitor for display at a second frequency of the first frequency, it is necessary to use two different memory units. While the video signal is being written into the first memory unit at the first frequency, the video signal is being read from the second memory unit at the second frequency. Similarly, while the video signal is being written into the second memory unit at the first frequency, the video signal is being read from the first memory unit at the second frequency. An example of such a device is given below.
These known systems, however, require that the second or output frequency be equal to, or a multiple of, first or input frequency so that the frequency of the output video signal will be synchronized to the frequency of the input video signal. As a result, such previously known systems could not be used to display a composite video image of two or more asynchronous video signals on a monitor.